Metallic impurities such as Fe, Cu and Ni are always present in silicon materials for solar cells, causing carrier recombination and reduced efficiencies. However, such impurities can be removed during cell fabrication, through a process known as impurity ‘gettering’, whereby the impurities are trapped in a region of the device where they have less impact. In industrial silicon solar cells this has traditionally been achieved via phosphorus diffusion, which forms the p-n junction for a standard p-type solar cell, and which also causes the impurities to segregate into the heavily phosphorus doped region. However, such phosphorus gettering is often ineffective in high efficiency n-type solar cells, in which the phosphorus diffusion is either restricted to a small fraction of the device surface, or entirely absent.
This project will develop new methods for gettering impurities in solar cells, based on the recent discovery in our laboratories that dielectric films such as silicon nitride and aluminium oxide, and also poly-siliocn layers, can also be very effective gettering layers. Ion implantation will be used to introduce known quantities of specific metallic impurities into silicon wafers for subsequent gettering experiments. Multicrystalline wafers with naturally occurring impurities will also be studied. Solar cells incorporating these new gettering techniques will also be fabricated in the laboratories at ANU. The project has the potential to lead to new industrial solar cell processes.
The project will be largely experimental, with some numerical modelling of gettering processes, and will be supervised by Prof Daniel Macdonald and Dr AnYao Liu. The project will involve collaboration with leading research groups at the University of Manchester and the University of Warwick in the UK.